The deflection of a tire is, by definition, its radial deformation, or its variation of radial height, as it passes from an unloaded inflated state to a statically loaded inflated state, under nominal conditions of pressure and load. It is expressed in the form of a relative deflection, defined as the ratio of this variation of the radial height of the tire to half the difference between the outside diameter of the tire and the maximum diameter of the rim measured on the rim flange. The outside diameter of the tire is measured statically in an inflated unloaded state at nominal pressure.
Although not limited to this use, the invention is more particularly described in respect of a tire with a radial carcass reinforcement, used on an airliner, and having a nominal inflation pressure of 17 bar, a nominal static load of 34 tonnes and a maximum speed of 380 km/h.
In the text below, the following expressions are given the meanings indicated:
“Radial plane”: a plane containing the axis of rotation of the tire.
“Equatorial plane”: the plane perpendicular to the axis of rotation of the tire, passing through the middle of the tire tread.
“Radial direction”: a direction perpendicular to the axis of rotation of the tire.
“Axial direction”: a direction parallel to the axis of rotation of the tire.
“Component X radially inward/outward of Component Y”: the radial distance of component X from the axis of rotation of the tire is less than/greater than, respectively, the radial distance of component Y from the axis of rotation of the tire.
“Component X axially inward/outward of Component Y”: the axial distance of Component X from the equatorial plane of the tire is less than/greater than, respectively, the axial distance of Component Y from the equatorial plane of the tire.
“Inside of the tire”: the inside of the cavity of the tire on which the inflation pressure acts.
The architecture of such an airplane tire is described for example in patent EP 1 381 525.
Such a tire comprises a tread designed to come into contact with the ground and connected by two sidewalls to two beads, each bead providing the connection between the tire and a wheel rim.
The tire also comprises a reinforcing structure consisting of a crown reinforcement radially inward of the tread, and a radial carcass reinforcement radially inward of the crown reinforcement.
The radial carcass reinforcement of an airplane tire usually contains a plurality of layers of reinforcing elements which are parallel with each other and form, with the circumferential direction, an angle of between 85° and 95°. The layers of reinforcing elements of the carcass reinforcement, which are termed the carcass reinforcement layers, are anchored, in each bead, to at least one circumferential reinforcing element or bead core. The carcass reinforcement layers usually comprise at least one so-called inner layer, which is wrapped around the bead core in a direction proceeding from the inside to the outside of the tire to form a turn-up that terminates at an end, and at least one so-called outer layer, which is wrapped around the bead core in a direction proceeding from the outside to the inside of the tire and axially outward of all the inner layers and their respective turn-ups, within the sidewall.
The reinforcing elements of carcass reinforcement layers, for airplane tires, are usually cords made of threads of textile filaments, preferably aliphatic polyamides or aromatic polyamides.
The mechanical properties in extension of textile reinforcing elements (modulus, elongation and force at break) are measured after prior conditioning. “Prior conditioning” means that the reinforcing textile elements are stored for at least 24 hours, before being measured, in a standard atmosphere according to European Standard DIN EN 20139 (temperature de 20±2° C.; hygrometry 65±2%). Measurements are made in a known manner using a ZWICK GmbH & Co (Germany) type 1435 or type 1445 traction machine. The textile reinforcing elements are pulled on an initial length of 400 mm at a nominal speed of 200 mm/min. All results are averaged over 10 readings.
The inner layer axially nearest the bead core is normally separated from the turn-ups and outer layers, which are axially outward of it, by at least one polymeric bead filler compound which is adjacent to and radially outward of the bead core.
Regarding the polymeric bead filler compound, the “modulus of elasticity” means a secant modulus of extension at 10% deformation and at room temperature. The modulus measurements are performed in traction according to Standard AFNOR-NFT-46002, September 1988: the measurement is performed on the second elongation (i.e. after an accommodation cycle) to give the nominal secant modulus (or apparent stress, in MPa) at 10% elongation (normal temperature and hygrometry conditions according to Standard AFNOR-NFT-40101, December 1979).
The radially outermost point of the polymeric bead filler compound, beyond which in the radial direction the carcass reinforcement layers are each coupled to their neighbour, is called the coupling point.
For the purposes of the invention, the coupling between two adjacent carcass reinforcement layers is characterized by a distance between their respective neutral fibres less than or equal to twice the diameter of the cross section of a reinforcing element forming part of the carcass reinforcement layers.
The location of the ends of the turn-ups and the coupling of the turn-ups to each other and to the adjacent carcass reinforcement layers, radially outward of the coupling point, ensures the anchoring of the turn-ups under the very severe load, pressure and speed conditions of such a tire.
In use, the mechanical rolling stresses cause cyclical flexing of the tire, which rolls up and down on the rim flanges.
In the following text, the expression “rim flexing area” denotes the part of the tire whose outer boundary rolls up and down on the rim flange, adopting its geometry in the contact area under the combined action of pressure and load.
The cyclical flexing generates, in the polymeric materials of the rim flexing area, and especially in those immediately adjacent to the turn-up ends, stresses and deformations which may in time degrade the tire and necessitate its replacement.
The cyclical flexing also generates, in the parts of the carcass reinforcement layers situated in the rim flexing area, variations of curvature combined with variations of tension. These variations of tension, particularly in the axially outermost layers, can be minimal, corresponding to compression which can cause failure of the material of the reinforcing elements of the layers and therefore deterioration of the tire.
Patent EP 0 599 575 has already described, in the case of a carcass reinforcement consisting of a plurality of layers of reinforcing elements, a way of preventing the risk of failure of the carcass reinforcement layers by locating the turn-up ends away from the rim flexing area. The rim flexing area, in the context of the patent cited above, is bounded by two straight lines perpendicular to the inner layers which are wrapped from the inside to the outside. The first straight line passes through the axially outermost point of contact, between the tire inflated at nominal pressure and unloaded, and the rim flange. The second straight line passes through the axially outermost point of contact, between the tire, inflated at nominal pressure and loaded to twice its nominal static load, and the rim flange. The turn-up ends are located either radially outward of the radially outermost straight line, or radially inward of the radially innermost straight line. The turn-up ends located radially outward of the radially outermost straight line create a relatively large bead thickness, in the rim flexing area, which is disadvantageous in terms of material costs. The turn-up ends positioned radially inward of the radially innermost straight line create a risk of premature deterioration of the tire by separation of the inner layers.
Patent EP 1 238 828 has also disclosed a solution for preventing the risk of failure of the carcass reinforcement layers, by reducing the number of turn-ups in the rim flexing area. This solution consists in having at least one inner layer with no turn-up, i.e. its end is positioned radially inward of the radially innermost point of the bead core. Under severe conditions of load, pressure and speed, the absence of turn-up can increase the risk of separation of the corresponding inner layer and therefore of the tire.